A typical printed circuit assembly (PCA) includes a section of circuit board material (i.e., layers of conductive and non-conductive material sandwiched together) and one or more components attached thereto. Examples of circuit board components include integrated circuits (ICs), resistors, capacitors, connectors and pins. Some circuit board components such as Ball Grid Array (BGA) devices, Chip-Scale Packaging (CSP) devices, Flip Chip devices and Multi-chip modules (MCMs) solder to metallic pads of circuit boards using Surface Mount Technology (SMT).
A typical metallic pad on the section of circuit board material includes a layer of copper, a layer of nickel and a coating of gold. Such a pad is typically formed using an electroless Nickel immersion gold surface finish process. The layer of copper is essentially an extension of the metallic etch (i.e., a trace) that runs between different locations of the section of circuit board material (e.g., to connect multiple devices together). The layer of nickel provides a durable mounting surface which has a high affinity for solder in order to form a solder joint with a corresponding contact of a circuit board component (e.g., during a circuit board manufacturing process such as reflow soldering). The coating of gold protects the layer of nickel against oxidation and from reacting with undesired contaminants.
After the circuit board components have been mounted to the section of circuit board material, the circuit board typically goes through a variety of tests (e.g., burn-in, vibration testing, extreme temperature testing, etc.). Occasionally, a circuit board may fail a test due to a defect in an electrical connection between a component and the section of circuit board material. One such defect is called a “Black Pad”. Such a defect can present itself as a pressure sensitive circuit board component having an intermittent electrical connection with the section of circuit board material (e.g., an intermittent connection between a metallic pad and a ball contact of a BGA device). Applying pressure to the circuit board component (e.g., pressing gently the BGA device toward the section of circuit board material) typically removes the intermittency and forms a solid and reliable electrical connection. However, once the pressure is removed from the component, the intermittency returns (i.e., the electrical connection between the metallic pad and the contact is again prone to intermittent failure).
One conventional approach to handling a “Black Pad” defect in a circuit board is simply to discard the circuit board. For example, in response to a failing circuit board in the field, a technician replaces the failing circuit board with a new circuit board. Later, if the failing circuit board is determined to have one or more “Black Pad” defects, the failing circuit board is thrown away.
A second conventional approach to handling a “Black Pad” defect within a circuit board is to simply (i) remove the circuit board component in order to expose the component mounting location having the “Black Pad” defect, (ii) melt and suck away solder from the component mounting location, and (iii) solder a new circuit board component to the component mounting location by using a conventional flux. For example, a technician lowers a nozzle, which is capable of applying heated gas, over the circuit board component in order to melt the solder joints and remove the component from the section of circuit board material. The technician then removes any excess of the original solder from the component mounting location. Next, the technician prints solder paste (e.g., a combination of solder and flux) over the component mounting location. Finally, the technician solders a new circuit board component to the component mounting location using the nozzle.